1. Field of the Invention
The present invention relates to an article of manufacture which is extremely valuable as a substrate for use in polishing materials, and especially for polishing semiconductor wafers. As set forth below, many other uses could be made of the novel article of manufacture according to the present invention.
More specifically, the present invention relates to a poromeric material comprising a piece or sheet of fibrous felt impregnated with a microporous elastomer wherein a majority of the fibers forming the felt are oriented primarily transverse to the surface of the article being used as a work surface, such as a polishing surface, bearing surface, etc., whereby the majority of fiber ends adjacent to the work surface, and preferably, the longitudinal axes of the fibers, form an angle of between about 45.degree. and 135.degree. with respect to the work surface. The present invention also relates to a process for making the novel article of manufacture.
2. Description of the Prior Art
Conventional substrates used for polishing and other operations for which the article of the present invention may be used include a non-woven piece or web of felt made from fibers such as polyester fibers, which is impregnated with a microporous elastomer, such as urethane, to produce a breathable, water-repellent structure. A typical example of a prior art structure is CORFAM poromeric material made by the E.I. du Pont de Nemours and Company (du Pont) and ULTRA-SUEDE poromeric fabrics of Kuraray Co., Ltd. These products are often used to make shoes, fabrics for clothing and upholstery, and have been used as substrates for polishing, as well.
In the prior art, the fibers of the impregnated felt are randomly oriented but generally are predominantly parallel to the major plane of the felt web. The web is ligated by forceably orienting sufficient fibers to a position substantially perpendicular to the plane of the web and/or interwoven so that a unitary, dense adherent structure is formed. During felting, some of the fibers are pushed into the perpendicular orientation, but subsequent calendering flattens most of the perpendicular fibers, and leaves them interwoven with the predominantly parallel fibers. Ligation typically is accomplished by punching the web with fine, barbed needles mounted in a conventional needle loom.
The following du Pont U.S. patents disclose various poromeric materials in which the fibers of the felt are randomly oriented generally parallel to the major plane of the felt sheet as described above and various methods of making them: U.S. Pat Nos. 3,000,757, issued Sept. 19, 1961; 3,067,483, issued Dec. 11, 1962; 3,100,721, issued Aug. 13, 1963; 3,180,853, issued Apr. 27, 1965; 3,208,875, issued Sept. 28, 1965; 3,284,274, issued Nov. 8, 1966; and 3,536,553, issued Oct. 27, 1970. These patents disclose examples of various synthetic fibers and elastomers which can be used to make the poromeric material, as well as various processing steps involved in making the poromeric material. The state of the art for making poromeric materials having the above-described prior art random fiber orientation is well developed and those of ordinary skill in the art would know of various substitutions that can be made in the prior art to make products having various characteristics.
The following U.S. patents in which I have been named a sole or joint inventor disclose the use of prior art poromeric materials for various polishing functions: U.S. Pat. Nos. 3,449,870, issued June 17, 1969; 3,499,250, issued Mar. 10, 1970; 3,504,457, issued Apr. 7, 1970; and 3,581,439, issued June 1, 1971.
It is known that poromeric material can be subjected to various surface treatments, such as buffing, napping or the like to alter the surface characteristics of the poromeric material. See, for example, du Pont U.S. Pat. No. 3,067,483 at column 2, lines 37-39, and U.S. Pat. No. 4,347,280, issued Aug. 31, 1982, disclosing an invention of which I was a co-inventor.
In conventional poromeric polishing pads, the fibers holding the pad together and supporting the soft elastomeric component contribute little to the polishing process. To the contrary, with pads containing fibers having a flat orientation at the polishing surface, the fibers actually impede and block the polishing process by trapping spent polishing slurry, small particles of the polished article which are removed in the polishing process, etc. Moreover, the fibers having a flat orientation at the pad surface shield from the workpiece a large proportion of the microporous elastomer which is the active polishing component. The sides of the fibers do not polish effectively.
Prior art polishing pads have to be replaced when the fibers at the surface of the pads trap foreign particles which actually may scratch the surface of the articles being polished. Where the articles are semiconductor wafers, even slight scratching renders them totally useless. Often, prior art poromeric polishing pads have to be replaced when less than 10% of the thickness of the pad has been used. This results in a significant waste and high cost of use. Moreover, a conventional pad cannot have its surface reconditioned and must be discarded when its surface becomes loaded or worn.
It is desirable to have as much of the porous elastomer available at the surface of the polishing pad as possible, since the elastomer is primarily responsible for the polishing action. However, if the reinforcing fiber content is reduced to expose more elastomer, the structural integrity of the pad and the elastomer is weakened to the point where it becomes so soft that it does not polish well. While the prior art reinforcing felt web with the random fiber orientation generally parallel to the plane of the web provides the poromeric elastomer with the required support to be useful, it does so at the expense of the desirable characteristics of the elastomer of softness, suppleness, flexibility and absorbency which are important for poromeric material, particularly when it is used in a polishing operation.
A feature of using poromeric materials for polishing is that the surface of the poromeric elastomer has a type of "pumping action" in which each small pore at or near the pad surface may be considered a miniature pump. The pressure of the workpiece passing over a pore compresses it, expelling spent polishing slurry and dross as the pad spins. The pore then expands and fills with fresh polishing slurry. With prior art polishing pads, the pumping action adjacent the surface of the pad deteriorates as the generally parallel fibers become loaded. Prior art pads become stiff and develop inflexible areas caused by dense or attenuated horizontal fiber bundles. Thus, the flexibility of prior art polishing pads is reduced, inhibiting the advantageous pumping action.
Polishing pads made of poromeric materials according to the prior art cannot be oriented to have polishing directionality, since the fibers are generally evenly oriented in the plane of the web. As conventional pads wear, they expose layer after layer of unlike structure, resulting in non-uniform polishing rates and characteristics which may adversely affect the polishing operation. Moreover, conventional polishing pads require a break-in period to reach their optimum polishing performance because certain surface effects resulting from the manufacture of the pads, such as surface coagulation and alteration during drying, buffing, splitting, and the like, act as barriers to the active pad. Additionally, as conventional pads wear, both fiber bundles and pieces of elastomer are subject to being pulled out in masses. This leaves gaping holes in the surface of the pad in which dross and chips collect, causing accelerated and uneven wear of the pad and ineffective polishing and often harmful scratching of a workpiece.
The present invention overcomes the undesirable aspects and disadvantages associated with prior art poromeric material. The poromeric material of the present invention represents a significant improvement over prior art poromeric material, particularly where it is used as a polishing substrate.